User manual MCINTOSH MAC4275

[. . . ] PREAMP OUTPUT · POWER AMP INPUT CONNECTIONS Great signal sensitivity together with freedom from strong signal overload is made possible by using MOS Field Effect Transistors in the FM RF circuits. 15. ULTRA STABLE LINEAR-PHASE PIEZOELECTRIC FM IF FILTERS Back panel connections provide the ability to insert room equalizers, reverberation units or other signal processing devices. 12. LOW NOISE ION-IMPLANTED J-FET OP-AMPS 16. The IF filters are permanently sealed and never require adjustment. The IF response curve is extremely selective because of its very steep slope either side of center and yet it has a linear phase characteristic. AUTOMATIC FREQUENCY LOCK New technology low noise operational amplifiers give lower noise and greater bandwidth than conventional devices. 13. [. . . ] TURN-ON TRANSIENT ELIMINATION CIRCUITS Speaker outputs are connected only after power supplies and circuits have stabilized eliminating turn on thumps or clicks. TECHNICAL HIGHLIGHTS The phono amplifier uses a new high technology integrated circuit operational amplifier. Its differential input stage has been optimized for low noise and low distortion performance. Open loop gain of this integrated circuit is 100, 000. With high open loop gain a large amount of negative feedback can be used around the phono amplifier to further reduce noise and distortion. The feedback network also provides precision RIAA frequency compensation. It uses 1% metal film resistors and 5% poly film capacitors. To achieve low noise performance it is essential that the feedback network be very low impedance. As a consequence, the preamplifier must be capable of operating as a power amplifier to drive this impedance. The actual power output capability of this preamplifier stage is more than 100 milliwatts, a great margin beyond that which is required. Electronic input selector switching uses field effect analog switches. The mechanical input selector simply switches small amounts of DC which turn the FET analog switches off or on. The circuit operates only during stereo reception, it switches automatically to monaural if the 19 kHz pilot tone is absent. In the PLL the internal oscillator operates at 228 kHz locked to the 19 kHz pilot tone. The 228 kHz feeds a 3 stage Johnson counter via a binary divider to generate a series of square waves. Suitably connected NAND gates and exclusive OR gates produce the tri-level drive waveform for the various demodulators in the circuit. The usual square waveforms have been replaced in the PLL and decoder sections by tri-level waveforms. These tri-level forms contain no harmonics which are multiples of 2 or 3. This eliminates frequency translation and detection of interference from the sidebands of adjacent stations since the third harmonic of the sub-carrier (114 kHz) is excluded and interference from SCA broadcasts since the third harmonic of the pilot tone (57 kHz) is excluded. Unwanted spurious audible components and phase jitter in the PLL with consequent intermodulation distortion are inherently eliminated by this technique. Additional advantages of the phase locked loop stereo demodulation are the elimination of inductors to minimize drift, integral lamp driving capability to indicate the presence of the 19 kHz pilot carrier, excellent channel separation over the entire audio frequency range, extremely low distortion, low output impedance, and transient-free mono/stereo switching. After multiplex detection, 19 kHz pilot and 38 kHz carrier suppression circuits are used to prevent tape recorder interference. It operates both by detecting ultrasonic noise and by sensing correct tuning of the detector circuit. [. . . ] With any further increase in distortion the POWER GUARD circuit will operate. This circuit limits the input dynamically so that the amplifier cannot be overdriven. POWER GUARD eliminates amplifier output clipping. POWER GUARD does not limit the dynamic range or the power output of the power amplifier. [. . . ]